In a liquid crystal display device of the guest-host type, a liquid crystal having dichroic dye dissolved therein is enclosed between two transparent substrates which have been subjected to an aligning treatment. FIG. 1 is a sectional view of a transmission type liquid crystal display device of the guest-host type. A liquid crystal 1 aligned in parallel with transparent substrates 3 turns to be arranged in a direction perpendicular to the transparent substrates 3 when a voltage is applied across transparent electrodes 4. At the same time, the direction of alignment of dichroic dyes 2 dissolved in the liquid crystal 1 changes too. The dichroic dye 2 has different light absorbance factors depending on whether the dichroic dye 2 is aligned in a direction perpendicular to or in parallel with the molecular axis. When a polarizer 5 has an absorption axis 7, the portion of the liquid crystal 1 having the voltage applied thereacross transmits light emitter from back light source 6, while the portion having no voltage applied thereacross does not transmit the light. The above fact can be utilized to display characters, patterns, pictures, etc. When a color filter for three additive primaries is disposed before or behind the liquid crystal panel, the liquid crystal panel can be used as a multicolor or full-color liquid crystal display device.
There was such a problem that, when the liquid crystal panel having the dichroic dye dissolved in the liquid crystal was used over a long period of time with the back light from back light sources being directed thereto, the dichroic dye gradually migrated until the dichroic dye was non-uniformly distributed, resulting in appearance of non-uniform color density and non-uniform luminance distribution on the display screen. It has been experimentally confirmed that, in this case, there is a correlation between the direction of migration of the dichroic dye and the direction of alignment of the dichroic dye.
FIGS. 2 and 3 illustrate the relationship between the direction of rubbing the transparent substrates and the direction of migration of the dichroic dye under a homogeneously aligned condition. In an initial state as shown in FIG. 2, the dichroic dye 2 is uniformly dispersed in the liquid crystal 1. When the back light from the back light source 6 is continuously radiated in a direction 8, the dichroic dye 2 migrates in direction opposite to the rubbing direction of the transparent substrate which is irradiated with the back light, and a non-uniform distribution of the dichroic dye 2 takes place, with the result that the dichroic dye 2 is distributed more in one area but less in the other area. In FIGS. 2 and 3, 9 designates the rubbing direction of the front transparent substrate, and 10 designates the rubbing direction of the back transparent substrate. It has been found out that the direction of migration of the dichroic dye 2 is determined only by the rubbing direction of the irradiated transparent substrate 3 and is not affected by the force of gravity, etc. Further, the migration of the dichroic dye 2 is a phenomenon due to excitation by the light, and such a phenomenon does not occur without irradiation with the light. Irradiation with the light causes migration of the dichroic dye 2 in one direction thereby producing, in the display screen, an area where the dichroic dye 2 is distributed with higher density to provide a low transmittance, and an area where the dichroic dye 2 is distributed with lower density to provide high transmittance. As a result, non-uniform color density and non-uniform luminance distribution appear on the entire display screen, thereby greatly degrading the quality of display.